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Homework Help: RC Circuit charging capacitor

  1. Mar 28, 2010 #1
    1. The problem statement, all variables and given/known data
    A 1234. pF capacitor and a 5.6 × 106 ohm resistor are connected in series to 78. volts EMF. Approximately how long does it take the capacitor to become almost fully charged?


    2. Relevant equations
    Vf=Vo(1-e^(-t/RC))


    3. The attempt at a solution
    I'm not sure if the equation I'm using is correct. I know there is one that has Q in it but couldn't find it. If there is Q in it how do I link it to volts? Through C=Q/Z?
     
  2. jcsd
  3. Mar 28, 2010 #2

    kuruman

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    You can use the equation you have. Just think, what will the voltage across the capacitor be when it is fully charged?
     
  4. Mar 29, 2010 #3
    Ahh..okay. But wouldn't Vo be 0?
     
  5. Mar 29, 2010 #4

    kuruman

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    Look at your expression. If V0 were zero, then the voltage would be zero at all times and the capacitor would never charge. Here, V0 does not represent the "voltage at time t = 0". It represents the constant emf provided by the battery. What happens to your expression if you let time t go to infinity?
     
  6. Mar 29, 2010 #5
    The equation that I have is Vc(t)=Vs(1-e^-t/RC)

    Where Vc is the voltage across the capacitor
    Vs is the voltae of the supply

    Hope this helps.
     
  7. Mar 29, 2010 #6
    Now I have a question:

    Im working with a signal generator RC circuit. And im trying to work out t=RC to workout the voltage waveform across the capacitor with the above mentioned formula.

    This is a very basic formula, but following a few of them in a text book has thrown me.

    eg.
    100 k Ohm
    0.001 mirco F
    This is what is in the textbook
    t=RC=(100k)(0.001mF)=100millisecond
    Now to me this is should be
    t=RC=(100000 Ohm)(0.000000001 F)=0.0001 sec

    I seem to be missing a decimal place each time. Can someone help please?
     
  8. Mar 29, 2010 #7

    kuruman

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    This is correct as it stands.

    1 mF (one millifarad) = 10-3F

    1 μF (one microfarad) = 10-6F

    Are you perhaps confusing the two?
     
  9. Mar 29, 2010 #8
    The mF that I have in there is microfarad. I did not have the mu sign.

    And as it is I dont get that value on the calculator.

    I get 0.0001sec
     
  10. Mar 29, 2010 #9

    kuruman

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    Then you are correct and

    RC=(100k)(0.001mF)=100millisecond

    is also correct. In your case the capacitance is in microfarads and in the other case the capacitance is in millifarads.
     
  11. Mar 30, 2010 #10
    Thank you for your help.

    What I have done was made a mistake where it should be mircosecond not millisecond.
     
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